ENERGY, PHOTOCHEMICALS AND BRIGHTNESS 303 



lichtempfindliche Thiere das Maximum der Reizwirkung 

 im Gelbgriin liegt, dass sie mit anderen Worten lichtemp- 

 findliche Substanzen besitzen die dem Sehpurpur der 

 Wirbelthieraugen ahnUch sind." Pfeffer (1906, p. 175) 

 maintains that " the relative efficiency of the different rays 

 is not the same in all plants," and Verworn says (1889, 

 p. 60), " Es hat sich herausgestellt, dass die meisten Protis- 

 ten nur auf bestimmte Farben, d. h. Strahlen von bestimm- 

 ten Wellenlangen reagiren, welche durchaus nicht fur alle 

 die gleichen sind." It is therefore evident that with ref- 

 erence to the reactions of the lower forms there are con- 

 tradictory opinions as to the efficiency of the different rays 

 of light. The same may be said in regard to animals with 

 image-forming eyes. 



Let us review the more important of the experiments 

 which have led to these contradictory opinions and try to 

 formulate the conclusions to which they lead. 



In this review we shall first attempt to ascertain the 

 efficiency of different parts of the spectrum in producing 

 reactions in various plants and animals, then we shall 

 compare this with the distribution of energy in the spec- 

 trum, with the distribution of brightness as judged by the 

 human eye, and with the distribution of actinic or photo- 

 chemical effect. 



Sunlight, as is well known, consists of ethereal vibrations 

 composed of waves varying in length from approximately 

 390'''' to 760''''. Aside from varying in length, light waves 

 may also vary in amplitude, and then there may be innu- 

 merable combinations of waves of different lengths. These 

 three different physical characteristics of light are said to 

 produce different specific subjective sensations in man, 

 known respectively as color-tone or hue, brightness or 

 shade, tint or saturation. The subjective sensations are 

 in all probability in some way associated with the effects 

 of light on chemical changes in the retina. 

 __ Monochromatic light, or light having a fixed color-tone, 

 consists of waves which are equal in length. In accord 



